set.ts

type NotObject<T> = T extends object ? never : T;

type IndexedFlat<T> = {
  [key in string]: NotObject<T>;
};

type IndexedNested = {
  [key in string]: object;
};

type Indexed<T = unknown> = IndexedFlat<T> | IndexedNested;

function isIndexedObject(obj: Indexed | unknown): obj is Indexed {
  return typeof obj === "object";
}

function isString(str: unknown): str is String {
  return typeof str === "string";
}

function set(
  object: Indexed | unknown,
  path: string,
  value: unknown
): Indexed | unknown {
  if (!isIndexedObject(object)) {
    return object;
  }

  if (!isString(path)) {
    throw new Error("path must be string");
  }

  let segments = path.split(".");
  if (object[segments[0]]) {
    object[segments[0]] = value;
  }
  let traversePath = (): Indexed => {
    return segments.reduce((acc, val, idx, keys) => {
      let next = keys[idx + 1];
      if (next) {
        keys.shift();
        acc[val] = traversePath();
      } else {
        acc[val] = value;
      }
      return Object.fromEntries(Object.entries(acc).slice(0, 1));
    }, {} as Indexed);
  };

  object = {...object, ...traversePath()};
  return object;
}

console.log(set(3, "foo.bar", "baz")); // 3
console.log(
  set({foo: 5}, "bar.baz.bla", 10) //  { foo: 5, bar: { baz: 10 } }
);